The ATM system has a broadly accepted set of standards which ensure compatibility of ATM networks and their components. The book "ATM (broadband-ISDN) a technical overview", published as publication GG24-4330-00, can be consulted in order to gain familiarity with the ATM system.
The ATM standards define the following basic characteristics of ATM communication:
All digital information is converted into cells (henceforth simply "cells"). These cells are transferred via a transmission medium.
Each cell has a fixed data length. The cell consists of a 5-byte header and a 48-byte information field. One part of the header carries data which is termed a "Virtual Path Identifier" (henceforth the "VPI"). Another part of the header carries data termed a "Virtual Channel Identifier" (henceforth the "VCI").
FIG. 1 (prior art) illustrates the parts of the header recognised as belonging to the VPI and VCI. At certain points in the ATM network, the region of the header labelled "GFC" (Generic Flow Control) is also interpreted as part of the VPI.
The path to be followed by a cell through an ATM network is defined by a series of "look-up tables" which are prepositioned in the switching points of the network. These look-up tables contain routing information to be consulted when a cell arrives at the switch point.
The ATM network has some switching points called "Virtual Channel Switches". A "Virtual Channel Link" is the link between two successive virtual channel switches in the ATM network. A "Virtual Channel Connection" (VCC) is a route through the ATM network consisting of a particular set of virtual channel links. An example of such a virtual channel connection is the route from the point of origin of a communication in the ATM network via two or more virtual channel links to the destination of the communication in the ATM network. It should be clearly understood that the virtual channel "connection" here refers to a route through the ATM network and not to the action of making a connection.
A switching action performed at a virtual channel switch in the ATM network is made in dependence on the data in both the VPI and VCI fields of the cell's header.
The ATM network also has some switching points called "Virtual Path Switches". A "Virtual Path Link" is the link between two successive virtual path switches in the ATM network. There may be several virtual path switches within one virtual channel link. The "Virtual Path Connection" (VPC) is the route through several virtual path switches followed by a particular cell. A switching action performed at a virtual path switch in the ATM network is made in dependence on only the data in the VPI field of the cell's header. In short, a VPI bundles several VCIs.
The prior art FIG. 2 illustrates the differences in the VP and the VC switching using a layer model. The lowest level of the model consists of the physical layer, i.e., the layer on which signals are exchanged. The virtual path connection layer (ATM VP) can be regarded as being mounted on the physical layer. A virtual path connection is marked VPC. The virtual channel connection layer (ATM VC) can be seen as third layer. A virtual channel connection is marked VCC. The origin of the cells in the simplified network of FIG. 2 is marked ORIG and their destination DEST. FIG. 2 recalls that virtual channel switching and virtual path switching are separate layers in the ATM hierarchy, even though some switches may act as either virtual channel or virtual path switch. However, the standard operation of a VC/VP foresees no means to cross from the VPC layer to the VCC layer except by terminating the entire VP and redirecting all VCs contained in it.
It may be equally instructive for an accurate understanding of the current invention to review the function of the look-up tables stored at the switching points:
A cell starts at its entry point into the ATM network with a certain value stored in the VPI data field in its header. When the cell reaches the first virtual path switch, this switch reads the VPI in the header. The value of the VPI is now used either directly or indirectly to derive the address to be located in the look-up table held in the virtual path switch. The data value found at that particular address in the look-up table is then put into the cell's header in place of the original VPI. The particular exit port from the virtual path switch out of which the cell is to be sent is also determined by a value stored at the same address in the look-up table. The cell is now sent from the first virtual path switch further through the ATM network, having had its VPI value changed by the virtual path switch. At each subsequent virtual path switch in the virtual path connection a similar switching action to this takes place, i.e. the value of the VPI in the cell on arrival at the switch is accessed in the look-up table, and the value stored in the look-up table under that address is inserted into the VPI data field in the cell's header prior to sending the cell further through the network. Thus the values stored in the look-up tables at the various switching points determine the cell's route through the ATM network.
Although the VPI value can be used as the address in the look-up table, other arrangements for finding a particular entry in the table may be contemplated. It is only important that the VPI value of the incoming cell reliably leads to the location of the stored information which tells the virtual path switch how to direct that cell. In particular. an example shown later in the description splits the look-up table described above into separate tables, to be used one after the other.
By definition, all cells having the same VPI are switched together at a virtual path switch, i.e. they are all sent on with the same new VPI independently of their VCI value. Therefore several cells with different values of VCI may follow the same virtual path connection. Thus the virtual path connection can be considered to be effectively a bundle of virtual channel connections.
In general, there will be one particular look-up table in the virtual path switch for each input port to the switch. Therefore a cell arriving at the switch at one particular input port with one particular VPI value in its header will not necessarily be sent out on the same output port as a cell with the same VPI value which arrives at the same virtual path switch on a different input port.
The switching action performed at a virtual channel switch involves consulting a look-up table which has entries accessed according to the value of the VPI and VCI data fields together. Comparing the possible number of VP addresses and of VC addresses, it is obvious that a VC switch requires larger look-up tables and thus vastly more memory capacity. In practice, a VC switch requires about thousand times more memory space than a VP switch in the same network.
There is in fact nothing in the ATM standard which prevents one switching point in the ATM network acting as both a virtual path switch and a virtual channel switch. Such dual function nodes in the ATM network fall within the scope of the present invention. For simplicity of explanation however, this description will deal only with switches which serve one of these functions.
The ATM standards for the VPI and VCI are given in "CCITT Recommendations I. 361 and 363".
ATM switching technology is also disclosed in numerous patents and patent applications. U.S. Pat. No. 5,239,537 for example describes means and methods to substitute a corrupted VP by an alternate VP. The U.S. Pat. No. 5,271,010 to Miyake describes a converter for converting the VPI and a VCI, i.e., the full 28 bit address attached to the header of an ATM cell. The converter comprises a plurality of identifier comparator units and a controller. Each of the identifier comparator units has an input identifier memory for storing an identifier attached to an ATM cell and a comparator for comparing the identifiers of an incoming ATM cell with the identifiers stored in the input identifier memory.
In view of the known ATM standard and technology, it is seen as object of the invention to provide means for selectively accessing cells having the same VPI but different VCIs. More particular, the invention aims at providing a method and a device for adding and dropping cells with a particular VCI from a VPI stream at a VP switch without having to terminate the VPI for all cells at this VP switch.